Purpose:
The goal of this study was to determine the effects of different ambient illumination levels on dopaminergic signaling in the retina and on the susceptibility of the mouse eye to lens-induced myopia. Previous studies have shown photopic lighting to protect against myopia in both controlled animal studies and correlational human population studies. Photopic lighting was tested as well as scotopic lighting since rods may be needed for emmetropization (Park et al IOVS 2014).

Methods:
Male C57BL/6J mice were exposed to photopic (15,000 lux, n=38), mesopic (50 lux, n=36), or scotopic (0.005 lux, n=38) lighting during the light phase of a 12:12 hr light cycle, starting at postnatal day 23 (P23). At P28, half the mice received head-mounted monocular lens defocus (-10D). Retinas were enucleated at P36 and analyzed for dopamine and DOPAC levels via HPLC. At each time point, the refractive error, corneal curvature, and ocular parameters of the mice were measured. The DOPAC/dopamine ratios were calculated as a measure of dopamine turnover.

Results:
After two weeks of exposure to photopic, mesopic, or scotopic light there was no myopic shift (OD minus OS) in the refractive development of the control animals. Mice with lens defocus under mesopic light had significantly larger myopic shifts (normalized to P28, -4.741±0.608; p<0.005) by P34 compared to mice exposed to photopic (-2.604±0.544) or scotopic (-1.807±0.608). Additionally, in a subset of mice, the difference in DOPAC/dopamine ratio between the lens defocused and opposite eyes was significantly increased in mice exposed to scotopic light levels (0.022±0.007); decreased in mice exposed to mesopic light levels (-0.020±0.003; p<0.001); and showed no significant change in mice exposed to photopic light (0.002 ±0.003). No significant differences were found in corneal curvatures or axial lengths.

Conclusions:
While photopic and scotopic light levels were protective against lens induced myopia with increases in dopamine turnover, mesopic light levels increased the development of lens-induced myopia with a decrease in dopamine turnover. This implies that high and low intensities of light may prevent myopia, while intermediate intensities, similar to indoor lighting, promote myopia.